1. Field of the Invention
This invention relates to the production of a high purity talc from ore including a high proportion of undesirable impurities, such as asbestos in the form of tremolite.
2. Description of the Prior Art
High grade talc in the form of powder has found wide acceptance, for example as a grain coating agent, as a filler in paper, plastic and rubber, and in the cosmetics industry for application to human skin. For such uses, it is desirable to have the talc as free as possible of any irritating particles. In several instances, the use of talc as talcum-powder or as a grain coating agent has been pointed out as a health hazard when ingested or inhaled; however, it has been hypothesized that the impurity tremolite in the talc is the harmful agent. Tremolite is known as a potential occupational hazard. Tremolite, as well as other forms of asbestos, are described by the American Conference of Governmental Industrial Hygienists as occupational carcinogens. Accordingly, a process for removing tremolite from talc would be desirable. Certain talc ores, such as, for example, those found in the region of Gouverneur, New York and certain California talc ores, contain a substantial proportion of tremolite. Such ores would therefore benefit from an effective process for removing tremolite from talc.
Talc is hydrated magnesium silicate, Mg.sub.3 [Si.sub.4 O.sub.10 ] (OH).sub.2, a layer type lattice with relatively well satisfied valence bonds within the layer lattice and is relatively inert in many environments. Its platy, inert type of structure, has made it a safe, useful powder for dusting or coating surfaces. Tremolite, on the other hand, is a fibrous or bladed type of amphibole, having the formula Ca.sub.2 Mg.sub.5 (Si.sub.8 O.sub.22) (OH).sub.2, has less saturation of the surface bond, is more reactive, and has been a concern for health reasons, as pointed out above.
Many physical processes for separating talc from tremolite and other undesirable impurities, such as dolomite, calcite, quartz, chlorite, mica, etc. have been tried and have been effective, albeit in many cases costs of processing the ore have been high. Selective mining and/or hand sorting of lumps of ore which can be distinguished visually can be done and aids to visual sorting, such as fluorescent effects, have also been used. See, e.g., U.S. Pat. No. 3,837,582 to Baak. On the other hand, when the talc and the tremolite in the ores are very finely disseminated, physical separation processes are less effective and satisfactory amounts of tremolite are not removed.
It is known that leaching of talc with a strong acid, such as a mineral acid, can improve certain of its physical properties such as its brightness, wettability and surface area. See, e.g., U.S. Pat. No. 2,576,653 to Thomas which describes mineral acid treatment of a slurry of finely ground particles of a clay, such as talc, under broadly defined conditions which can include open vessel heating, refluxing, or heating in a closed vessel under steam pressure. Other patents which may be of some interest in this regard are U.S. Pat. No. 3,533,821 to Lundquist, U.S. Pat No. 3,271,323 to Whittmore, U.S. Pat. No. 2,574,895 to Stecker, U.S. Pat. Nos. 1,598,255 and 1,598,256, both to Prutzman et al, and U.S. Pat. No. 89,438 to Sell. While benefiting certain physical properties of talc, as described, none of the foregoing references report a decrease in tremolite, nor are they in fact particularly concerned with tremolite-containing talc.